Rosat/asca Observations of a Serendipitous Bl Lac Object Pks 2316-423: the Variable High-energy Tail of Synchrotron Radiation

We present the analysis of archival data from ROSAT and ASCA of a serendipitous BL Lac object PKS 2316-423. Because of its featureless non-thermal radio/optical continuum, PKS 2316-423 has been called as a BL Lac candidate in the literature. PKS 2316-423 was evidently variable over the multiple X-ray observations, in particular, a variable high-energy tail of the synchrotron radiation is revealed. The X-ray spectral analysis provides further evidence of the synchrotron nature of its broad-band spectrum: a steep and downward curving spectrum between 0.1–10 keV, typical of high-energy peaked BL Lacs (HBL). The spectral energy distribution (SED) through radio-to-X-ray yields the synchrotron radiation peak at frequency νp = 7.3 × 10 15 Hz, with integrated luminosity of Lsyn = 2.1 × 10 44 ergs s. The averaged SED properties of PKS 2316-423 are very similar to those “intermediate” BL Lac objects (IBL) found recently in several deep surveys, such as Deep X-ray Radio Blazar, RadioEmitting X-ray, and ROSAT-Green Bank surveys. We suggest that PKS 2316-423 is an IBL though it also shows some general features of a HBL. Actually, this double attribute of PKS 2316-423 provides a good test of the prediction that an IBL object can show either synchrotron or inverse-Compton characteristics in different variability states. keywords: BL Lac objects: individual (PKS 2316-423) – X-rays: galaxies Beijing Astronomical Observatory and Beijing Astrophysics Center of National Astronomical Observatories, Chinese Academy of Sciences. A20 Datun Rd., Chaoyang District, Beijing 100012; E-mail: [email protected] International School for Advanced Studies, SISSA/ISAS, via Beirut 2-4, 1-34014 Trieste, Italy; E-mail: [email protected] 1